Electrical system for review

[mburch]

Hi all,

I've gotten far enough into the design of the electrical system for my
RV-7 that I feel like it's ready for others to take a look at it. It is a
fairly faithful adaptation of Figure Z-13/20, with a few changes. I've
combined the schematics with a writeup containing a high level description
of the architecture and design goals, and notes on specific choices I made
in laying it out, and posted it all as a PDF file on my website:

http://www.rv7blog.com/files/2007092...cal_system.pdf

If any of you experts have a chance to look it over and offer any
comments or criticism, I'd sure appreciate it. If you see something that
doesn't make sense in the diagrams or description, let me know and I'll
try to explain what I was thinking.

thanks,
mcb

[Ironflight]

Looks pretty good to me Matt. My only thought as I looked through it is that you might not be happy with the size of your aux battery. I am using a 4.5 aH, and that will just give me 30 minutes on my two-screen GRT and AHRS system. I tried a 3.0 Ah and only got about 9 minutes. I do replace the battery once a year - I have a convenient electronic parts store, so it is pretty easy, and relatively cheap. unless you have a severe space limitation, you might think of going up one size.

Paul

[JonJay]

I noticed the "double" fusing (60A and 40A E) on the Alternator and Battery main lines. Is this normal?
There are other unprotected lines in the system. Just curious why you chose the Alternator fusing at that location. Maybe I need to add a fuse on mine?

Obviously, you have a lot of extra redundancy for your electronic ignition, etc..., you still have a mag though. (good, in my opinion)

Your design looks very complete, and although really not that complex, a considerable amount of extra weight? (that's a question not assumption) I assume you are intending a lot of IFR to justify the redundancy? If not, you have a lot of extra parts, connections, etc... for gremlins to sneak in over time.

Just a comment - I believe an airplane should be flyable with a total loss of electrical power, from either or both systems. I assume you have appropriate back up instrumentation in case both fail.

Excellent write up and diagrams.

[gmcjetpilot]

Is this an electrical system for a RV or a Boeing 737?


I'm still looking at it, but do you have to throw 5 switches as normal procedures? 3 Master/Bat switches (Buss 1, Buss 2, Aux Bat), plus an electronic ignition switch and mag switch? Nothing wrong with this, but my comment / suggestion is add check-list/procedures for normal, non-normal and supplemental operations to the system description.

Are there critical pilot (switch) actions? Meaning, can throwing or not throwing switches out of sequence or not at all, cause a problem? In other words, can the pilot do something to really screw-up a perfectly good electical system?

What if you leave the electronic ignition switch on (even with masters 1, 2 & aux off)? Will it drain battery'(s)? If yes, what do you have help you not forget? (By the way I have this issue since the EI is hard wired to the battery.)


If you have complicated procedures (like throwing 5 switches, twice, every flight for one system), I'd suggest making and using a check-list religiously. The more complicated systems are, the more redundancy usually, but the need for pilot training, knowledge, currency & adherence to strict procedures goes up. Check-list are good for everyone and all planes, but a simplex plane (one bat/alternator) can get by with mnemonics or song-list. I use, MMM, CIGARS, GUMPS....

Tell you why I say this. An example, a very popular jet airliner has an electrical system switch that normally should never be touched, unless a check-list calls for it, only for a non-normal condition. However unknown to a crew (may be the engineers did not think of it either) this switch can disable the whole planes electical system, after a period of time, with some what subtle warnings, especially if they're ignored. No one knew this could happen, and this plane model was in service for many years, before this crew figured out a way to cleverly (carelessly?) defeat all the protection with one switch.


Nice and schematics/description are outstanding. I also noticed the double fuse for the 60A alternator. That's should be OK; my first plane's electrical system was like a 55 chevy or ford and worked fine. On the other hand all electric IFR kitplane are the norm now. Looks good to me; should work great, but the devils in the detail of good installation, which I'm sure will be as top notch as your documentation. Nice and thanks, I now have electical system envy!

I'll save this and if I ever have an all electric IFR RV, I'll steal and copy your ideas. Thanks for sharing.

[JonJay]

My first plane's electrical system was like a 55 chevy or ford and worked fine. .[/quote]


Mine still is!! But always nice to see what folks are doing.

[Walt]

I know how much fun it is to "play" with this stuff but it seems that it may be overly complex for the mission that the RV is really designed for. But if that's what you feel you need for redundancy than go for it. Like said above an emergency and standard procedure check list would seem to be in order and needed to effectively operate the system. You may have it all figured out now and have it memorized how it's all tied together but a year or two down the road when a fault occurs in flight you may have trouble remembering what to do!

A couple of things I noticed that maybe you haven't thought of are 1) You show an ANL 40 for the feed on the SD20; 2)If the Aux battery dies it won't be able to recharge because it won't be able to pull the relay, 3) Do you really want to have the starter locked out unless both ignition switches are on? I can think of a number of reasons to eliminate this. 4) Klaus emphatically recommneds again hooking the LSE to anything but the battery directly for spike protection.

[mburch]

Thanks for the feedback, everybody. Let's see if the forum's new multi-quote feature works...

Originally Posted by Ironflight

My only thought as I looked through it is that you might not be happy with the size of your aux battery.

I think you're right, Paul - after I built my aux battery box the other day, I wasn't terribly happy with the location and size I picked. I've now found a different battery (Panasonic high discharge rate job designed for backup power supplies) that I'm going to mount in a different spot. It's supposed to be able to run my worst case 4A load for 40+ minutes, which is better.

Originally Posted by JonJay

I noticed the "double" fusing (60A and 40A E) on the Alternator and Battery main lines. Is this normal?
There are other unprotected lines in the system. Just curious why you chose the Alternator fusing at that location.

The intent of the "double" current limiters is to protect the battery, two different ways: one saves the day if the alternator develops a catastrophic short, and the other one protects the battery if a short develops in the approximately two-foot-long, fat wire that penetrates the firewall and runs to the main bus fuse block. I could have used a single current limiter instead of two, but a faulted alternator would have also taken the main battery offline with that configuration.

By virtue of the nature of aircraft electrical systems, there are in fact a few unprotected wires. I've tried to note all of these with an asterisk, denoting that they should be constructed in such a way that they're as short as possible. If you see a wire that doesn't have a fuse, circuit breaker, or fusible link upstream and that doesn't have an asterisk on it, please tell me so I can fix my mistake!

Your design looks very complete, and although really not that complex, a considerable amount of extra weight? (that's a question not assumption)

That's certainly a fair criticism. Believe me, I drew some way more elaborate designs before I settled on this one. Although the number of extra schematic symbols seems excessive, I don't feel like there's an unacceptable amount of extra weight added. Fuse blocks and relays are pretty light, and my aux battery weighs about 5 lbs. There isn't any extra heavy-gauge wire since all the aux bus currents are low. I need to lose about 5 lbs anyway, so there's where I'll get the payload back!

Just a comment - I believe an airplane should be flyable with a total loss of electrical power, from either or both systems. I assume you have appropriate back up instrumentation in case both fail.

Absolutely.

Originally Posted by gmcjetpilot

Is this an electrical system for a RV or a Boeing 737?

Another fair comment!

I'm still looking at it, but do you have to throw 5 switches as normal procedures? 3 Master/Bat switches (Buss 1, Buss 2, Aux Bat), plus an electronic ignition switch and mag switch? Nothing wrong with this, but my comment / suggestion is add check-list/procedures for normal, non-normal and supplemental operations to the system description.

Sorry, I should have been more descriptive of my thinking here. Yes, there are three power switches, and two ignition toggles. I have added an aux bus master switch, but I have combined the functions of an E-bus alt feed switch and an aux alternator field switch into the Bus 2 Master switch. So really, I think I have the same number of switches as a typical Z-12 or Z-13 design. I just went back to the book to make sure and it seems like the same number to me.

I tried to make normal operation be as simple and carefree as can be: Turn on all three master switches, turn on the ignitions, crank up, make sure all the red lights are out, and go flying. And good input on the checklist: I always try to remember that I may not always be the PIC in this ship, so as the designer I have a responsibility to make the operation of the system clear to those who'll fly it other than me. Part of this is good "switchology" and labeling, and another big part is constructing clear checklists that spell everything out.

Are there critical pilot (switch) actions? Meaning, can throwing or not throwing a switch, out of sequence or not at all, cause a problem? In other words, can the pilot do something to really screw-up a perfectly good electical system?

I hope not - I've tried to think through all the possibilities. You can't do any damage if you move any of the bus master switches in the "wrong" sequence, since there shouldn't be anything order-dependent. Let me know though if you spot anything suspicious. And actually you could accidentally leave both the Bus 2 Master and Aux Batt Master switches completely off, and still go flying - you wouldn't have the aux battery or standby alternator online, and the E-bus alt feed path would be open, but all your stuff would come on and be powered by the main alternator. No different from a much simpler system, really - I'm trying to think in layers after all. Of course if you left the Bus 1 Master switch off, it wouldn't crank when you pushed the starter button, so you'd probably know something was up if you did that!

What if you leave the electronic ignition switch on (even with masters 1, 2 & aux off)? Will it drain battery'(s)? If yes, what do you have help you not forget? (By the way I have this issue since the EI is hard wired to the battery.)

Oh George, why do you tempt me... yes, it would run down the battery. So just for you, I designed a circuit that will warn me if I turn all the airplane power off but leave the ignition turned on. (see below for update)

Tell you why I say this. An example, a very popular jet airliner has an electrical system switch that normally should never be touched, unless a check-list calls for it, only for a non-normal condition. However unknown to a crew (may be the engineers did not think of it either) this switch can disable the whole planes electical system, after a period of time, with some what subtle warnings, especially if they're ignored. No one knew this could happen, and this plane model was in service for many years, before this crew figured out a way to cleverly (carelessly?) defeat all the protection with one switch.

That's sobering. But that's exactly why I wanted to put my drawings up for criticism... to force myself to think about how to avoid that kind of trap. I think I've got a way to warn the pilot of just about anything bad that could happen, either by component failure or pilot action, but I guess you never know.

I also noticed the double fuse for the 60A alternator.

Yah, see above and look closely at the schematic, and I think you'll see the intent. Or maybe not, let me know if it still looks unnecessary.

I'll save this and if I ever have an all electric IFR RV, I'll steal and copy your ideas.

You and everyone else are welcome to it!

Since the forum places a limit on the length of a single post, this will be continued in part 2...

[mburch]

Continued...

Originally Posted by Walt

You show an ANL 40 for the feed on the SD20

Yep - I'd use an ANL20 or 30 there but B&C doesn't sell them smaller than a 40-amp job. I called and was told that an ANL40 is what they ship an with the SD-20 STC kit for Bonanzas and 210's. Their point was that if the alternator takes a dump and tries to short the battery to ground, you'll get hundreds of amps for a split second, so worrying about a few tens of amps' worth of fuse rating is kind of moot.

If the Aux battery dies it won't be able to recharge because it won't be able to pull the relay

Good eye, I hadn't noticed that. Well, I do have a way to recharge it on the ground (molex connector on page 5) and a way to detect this has happened - if only the Aux Batt Master switch is on, and the #1 EFIS doesn't turn on, then the aux battery is so dead that it can't close its relay. I actually have modified my drawings (see below) to remove one of the two relays in the aux battery area, replacing it with a diode a la Lightspeed's recommendation. I may in fact look for a way to get rid of the other relay to avoid the issue you pointed out.

Do you really want to have the starter locked out unless both ignition switches are on? I can think of a number of reasons to eliminate this.

Yah, I thought better of it and removed it. I'll just use a guarded pushbutton or keyswitch (start only) instead. Thanks for keeping me grounded, pardon the pun.

Klaus emphatically recommneds again hooking the LSE to anything but the battery directly for spike protection.

Hmm. Well, I have it hooked to two batteries, through fuses. Other than the diode, you can't get much more direct than that... I think.

Thanks everybody for your input, I really appreciate it. I've incorporated the feedback and a couple news ideas and updated my drawings, which you can find here.

(Edit: Bumped the file revision overnight to correct a dumb mistake on the aux bus sheet)

thanks,
mcb

[dan]

What's your average mission, and why are you going with so much redundancy?

[mburch]

Originally Posted by dan

What's your average mission, and why are you going with so much redundancy?

A mix of local flying and long cross country trips. As to why do it, probably a combination of "because I feel like I need it", "because I enjoy this stuff", and "because I can".

mcb